To move it to another Mac, copy it over a local network or place it on a removable drive such as a flash drive or external hard drive that you then connect to another Mac. The Yosemite license allows you to install that single copy on multiple computers.
Begin the installation
Once the download is complete the installer will launch and display the Install OS X window, which contains a Continue button. Click that button and you'll be asked to agree to the license agreement twice. In the next screen you'll find a thumbnail image of your startup drive, indicating that the installer is willing to update your current operating system.
If you have access to other partitions or drives attached to your Mac, a Show All Disks button will also appear. To chose a different volume or partition to install Yosemite on, click this button, select the destination you'd like to use, and click on Install. In the window that appears enter your user name and password.
A pre-installation process begins where the installer gets the Mac and its destination volume ready for the job at hand. During this time you'll see a gray progress bar. When the process finishes you'll be told that the Mac will automatically restart in 30 seconds. You're welcome to click the Restart button if you can't wait that long.
The Mac then reboots, displays the Apple logo along with another progress bar. Then the Install OS X window appears with a blue progress bar and the words "Installing on [nameofvolume]" with an estimated remaining time appearing below the progress bar. Thrilling though this may be to watch, you may wish instead to take a break as the installation can take more than 15 minutes. When this is all complete the Mac will restart and you'll be prompted to start configuring the OS.
Performing a clean install
After the Mac boots you'll see a variety of configuration windows. Which you see depends on whether you're installing Yosemite over an existing Mac OS installation or you're installing it on a drive that didn't previously hold a copy of the Mac OS (this is termed a clean install). Here's what you'll find when preforming the latter.
The first window asks you to choose the country where you reside. This is to establish time zone as well as location. At the bottom of the window you'll see instructions for activating Mac OS X's VoiceOver screenreader feature, which allows those with visual impairments to operate the computer. If you have such an impairment, you can get started with VoiceOver by pressing the Escape key.
You're then tasked with choosing the kind of keyboard layout you use; a Wi-Fi network (if your Mac isn't jacked into an Ethernet connection); and whether you want to migrate data from a Mac, Time Machine backup or startup disk, or a Windows PC. Choose to migrate data and Yosemite's Migration Assistant launches and walks you through the process.
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